One of the liquid crystal display (LCD) manufacturing steps includes a bonding step in which sheet-shaped optical members each including a polarizing plate are bonded to both surfaces of a liquid crystal cell including a pair of substrates and a liquid crystal layer interposed therebetween. In general, such optical members include a polarizing plate and an adhesive or pressure-sensitive adhesive layer formed on one side of the polarizing plate.
A method used for bonding such optical members to a liquid crystal cell includes feeding the optical members and the liquid crystal cell in the same direction and press-boding the optical members to the liquid crystal cell, while stacking them. Specifically, a known method includes: feeding the liquid crystal cell and the optical members between a pair of rollers opposed to each other and placed perpendicular to the direction of feeding of the liquid crystal cell and the optical members (a guide roller for feeding the liquid crystal cell and a bonding roller for press-bonding the optical members to the liquid crystal cell), while stacking them; and press-bonding the optical members to the liquid crystal cell. In this case, the adhesive or pressure-sensitive adhesive layer formed on the optical member may be covered with a peelable release film, and the optical member may be bonded to the liquid crystal cell by the above step, while or after the release film is peeled off.
Examples of such a liquid crystal display device manufacturing method include a piece-by-piece manufacturing method in which optical members obtained by previously cutting into predetermined sizes are bonded to a liquid crystal cell by the above method; and a continuous manufacturing method in which optical members fed from continuous rolls are sequentially bonded to a liquid crystal cell, while cutting the optical members with the carrier films left uncut (see Patent Document 1).
In particular, the continuous manufacturing method is significantly advantageous in terms of mass productivity or yield as compared with the conventional piece-by-piece method, because the step of continuously bonding the optical members to the liquid crystal cell is achieved. In that method, the optical members are preferably bonded under a certain tension for such a purpose as to suppress wrinkles or air bubbles in the process of bonding the optical members to the liquid crystal cell. In the method for continuously manufacturing a liquid crystal display device, which includes feeding a carrier film on which the optical member is placed, a tension has to be applied in the feed direction, that is, in the longitudinal direction of the carrier film, and also the bonding has to be performed while the optical film is pulled. Therefore, a tension is also preferably applied between the bonding roller and the optical member.
In the bonding of the optical member, slack in the optical member may cause a failure of the bonding. Since it may cause wrinkles or air bubble, the optical member has to be kept smooth, and therefore, the application of a tension to the optical member is still important. If the tension is too high, however, a serious problem with quality or display quality, such as a cause of film rupture or a photo-elasticity-induced retardation may occur.
For example, an elongated polarizing plate may be bonded under a tension applied along its longitudinal direction, and the pair of rollers may be used to apply a tension to the optical member.
The optical member typically includes a polarizing plate. Such a polarizing plate is generally produced by allowing a dichroic material to adsorb to a polyvinyl alcohol (PVA) film and stretching the film. It usually has absorption dichroism for electromagnetic waves including visible rays and has an absorption axis parallel to the stretching axis.
Patent Document 2 discloses a continuous manufacturing method using a set of continuous rolls including long optical members which have been previously produced according to the size of a liquid crystal cell. Specifically, the method includes: using a set of continuous rolls including long optical members having undergone slitting in the direction of the absorption axis of the polarizing plate into sizes corresponding to the short and long sides of a rectangular liquid crystal cell; cutting the long optical members fed from the continuous rolls, respectively, into lengths corresponding to the long and shot sides of the liquid crystal cell; and bonding the cut pieces to the liquid crystal cell. Such a continuous manufacturing method allows efficient bonding to both surfaces of a rectangular liquid crystal cell simply by cutting the long optical members fed from the continuous rolls into predetermined lengths.
As liquid crystal panels are increasingly used in liquid crystal televisions, they are increasingly required to have larger sizes and higher quality. Such high-performance liquid crystal panels use a high display quality mode such as a normally-black vertically aligned (VA) mode or a horizontal electric field (IPS) mode. To form such liquid crystal panels, optical members including polarizing plates each having an absorption axis parallel to a side of a rectangular liquid crystal cell need to be bonded orthogonal to each other to the rectangular liquid crystal cell.
In a laterally-long rectangular liquid crystal display, for a polarizing sunglass measure or the like, polarizing plates bonded to a liquid crystal panel preferably have a viewer side (upper side) absorption axis placed in the lateral direction, that is, the longitudinal direction, and a backlight side (lower side) absorption axis placed in the vertical direction, that is, the transverse direction.
Patent Documents 4 to 7 also disclose part of the continuous manufacturing method.
Prior Art Document
Patent Documents
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2009-61498
Patent Document 2: Japanese Patent No. 4406043
Patent Document 3: Japanese Patent No. 4307510
Patent Document 4: JP-A No. 2004-361741
Patent Document 5: JP-A No. 2004-333647
Patent Document 6: JP-A No. 2005-37416
Patent Document 7: JP-A No. 2005-37417